New Twiddle

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  w: 50,
  width: Ember.computed('w', function() {
    return Ember.String.htmlSafe(`width: ${this.get('w')}px`);
  }),
  mouseMove(e) {
    this.set('w', e.clientX - 10);
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

{{my-component}}

templates.components.my-component.hbs

<div style="background: #f60; height: 200px; {{width}}">
</div>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}